Cardiac pacing is delivered to patients to treat a wide variety of cardiac dysfunctions. Cardiac pacing is often delivered by an implantable medical device (IMD), which may also be equipped to provide cardioversion or defibrillation, if needed. The IMD delivers such stimulation to the heart via electrodes located on one or more leads, which are typically intracardiac leads.
In demand pacing, a pacing pulse is delivered when an interval expires without detecting an intrinsic depolarization of the heart. The interval may be referred to as an escape interval, and controls the minimum rate of depolarizations. In rate-responsive pacing, the escape interval may be varied based on the physiological needs of the patient, as indicated by one or more sensors. As an example, an activity or respiration sensor may indicate increased exertion or activity of the patient, which in turn indicates a need for an increased heart rate and shortening of the escape interval. The depolarization rate indicated by the output of the one or more sensors may be referred to as a sensor-indicated rate.
In cases in which a patient's atrioventricular conduction is compromised, the ventricular pacing may be delivered an atrioventricular (A-V) interval after detection of an intrinsic or paced atrial depolarization. The A-V interval may also be varied based on the output of the rate-response sensor(s). Such pacing of the ventricles may be referred to as ventricular tracking pacing, because the paced depolarizations of the ventricles track the rate of depolarizations of the atria.
Patients with heart failure are, in some cases, treated with cardiac resynchronization therapy (CRT). CRT is a form of cardiac pacing. In some examples, CRT involves delivery of pacing pulses to both ventricles to synchronize their contraction. In other examples, CRT involves delivery of pacing pulses to one ventricle to synchronize its contraction with that of the other ventricular, such as pacing the left ventricle to synchronize its contraction with that of the right ventricle.
Ventricular tachycardia (VT) and ventricular fibrillation (VF) are cardiac arrhythmias that originate in the ventricles of the heart. The ventricular rate during VT may range from 150 beats per minute (bpm) to 188 bpm, for example, and is relatively stable. The ventricular rate during VF is more rapid, and may be unstable or disorganized. VT may be treated by anti-tachycardia therapies, such as anti-tachycardia pacing or cardioversion. VF may be treated with defibrillation. Supraventricular tachycardia (SVT) is characterized by a rapid ventricular rate (e.g., 150 bpm to 188 bpm), but originates from outside of the ventricles, e.g., the atria or the atrioventricular node. In some cases, SVT does not require treatment.
Conventional IMDs for treating ventricular tachyarrhythmias, such as VT or VF, monitor the ventricular rate and determine whether the ventricular rate falls within one or more zones, e.g., a VT zone and/or a VF zone. An example VT zone is a zone of ventricular rates between and including 150 bpm and 188 bpm. An example VF zone includes rates greater than 188 bpm. Conventional IMDs apply anti-tachycardia therapy or defibrillation when the ventricular rate is within the VT or VF zone for a threshold number of consecutive or proximate depolarizations. Conventional IMDs do not allow a sensor indicated pacing rate to exceed the lower bound of the VT zone, or a ventricular tracking rate to track atrial rates above the lower bound of the VT zone.